Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD |
Pluto Atmospheric Observations: NH Post-Encounter Phase, 1 Aug 2015- TBD |
Jul 31 2015, 02:57 PM
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Senior Member Group: Members Posts: 1669 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
A neat paper by Jonathan Fortney shows this ratio to scale (approximately) with sqrt(Rp/H), with Rp being the planet radius and H the scale height. Both indeed decrease this effect for Pluto. If we assume the scale height of Pluto's atmosphere is 60km and the aerosols have the same height as the gas, then I was able to get a few numbers in the course of comparing various airmass equations. Earth would be about 39 airmasses in the horizontal and Pluto would be 6.4. These numbers would be doubled when looking at grazing incidence from space as in the NH images. I'd still like to come up with a formula for an isothermal atmosphere (exponential density decrease with height) by integrating the thin shell relationship over height and to compare this with the other formulations in Wikipedia. On the other hand, the isothermal case is within just a few percent of the homogeneous (constant density with height) case. To check the scale height and see why it is much higher than Earth, we might evaluate this expression for Earth and Pluto: H = kT/mg H is scale height T is temperature (a representative value since this varies with height) k is Boltzmann's constant m is molecular mass g is gravitational acceleration The Wikipedia link above shows this worked example for Earth: Taking T = 288.15 K, k = 1.3806488x10-13 J/K, m = 28.9644×1.6605×10−27 kg, and g = 9.80665 m/s2 yields H = 8345m Roughly speaking, if pluto has .07 Earth's gravity and the same T and similar m we'd get about 120km scale height. If the scale height is 60km, then the temperature would still end up being ~140K. So we can check how much the temperature increases with height over the surface value of 44K. There are other atmosphere posts in the Near Encounter thread as well (e.g. posts #1238 and #1252). -------------------- Steve [ my home page and planetary maps page ]
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Aug 12 2015, 06:11 PM
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Senior Member Group: Members Posts: 1669 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Offhand, the phase function peak is more typically ranging from 20-1000 for various solar system hazes (including Earth). This agrees with the figure in 'remcook's link. The larger the particles, the larger the peak. So Gennady's plot value of 6 might indeed benefit from being increased. If we consider exactly where the sun is behind the disk of Pluto, then the asymmetry of the annular glow may help a bit in determining the phase function if shadowing effects of Pluto on its atmosphere are also considered. LORRI's field of view though is a bit small though for the scattering angle to vary much over the image.
-------------------- Steve [ my home page and planetary maps page ]
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Aug 12 2015, 07:41 PM
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Senior Member Group: Members Posts: 4256 Joined: 17-January 05 Member No.: 152 |
If we consider exactly where the sun is behind the disk of Pluto If you mean the released LORRI post-encounter images, the sun is not behind Pluto. NH was 360 000 km from Pluto in the closer post-encounter frames, which is much farther than the Pluto-sun occultation distance (very roughly 50 000 km). Conversely, LORRI would only see a small fraction of Pluto during the occultation.The variation in brightness we see around the limb in those shots is mainly due to the presence of a very slim sunlit crescent, though presumably the haze is somewhat in shadow on the opposite limb. Edit: this assumes you meant "occulted by Pluto" when you said "behind Pluto". |
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